Alumino-silicate pigment and method of making same



Sept. 24, 1968 s. c. L ONS ET AL 3,403,041

ALUMINO-SILICATE PIGMENT AND METHOD OF MAKING SAME Filed Oct. 22, 1965 2Sheets-Sheet 1 H 0 --A-D'ELAMINATED CLAY AI(OH)3 no?o H2O DEWATER H2ODRY PULVERIZE (OPTIONAL) OPTIONAL QUENCH IN H20 COOL OVERSIZE -CLASSIFYPULVERIZE DRY I PULVERIAZE INVENTORS SANFORD C. LYONS BY JOHN H..CHAPMAN AT TORN EYS PERcE yT REFL Sept. 24, 1968 ALUMINO-SILICATEP-IGMENT AND METHOD OF MAKING SAME Filed Oct. 22, 1965 gGTANCE 2Sheets-Sheet 2 l 1 1 l 1 I I 500 20 4o 60 80 600 20 40 60 so 700WAVELENGTH IN MILLIMICRONS INVENTORS SANFORD C. LYONS JOHN H. CHAPMAN ATTORN EYS FIG. 2

United States Patent 3,403,041 ALUMINO-SILICATE PIGMENT AND METHOD OFMAKING SAME Sanford C. Lyons, Bennington, Vt., and John H. Chapman,Jeffersonville, Ga., assignors to Georgia Kaolin Company, Elizabeth,N.J., a corporation of New Jersey Filed Oct. 22, 1965, Ser. No. 501,2072 Claims. (Cl. 106-288) ABSTRACT OF THE DISCLOSURE An alumino-silicatepigment is prepared by (a) delaminating washed coarse kaolin andrecovering the delaminated fraction up to microns (E.S.D.), (b) mixingan aqueous slurry of the delaminated fraction with an aqueous slurry ofaluminum hydrate in the proportion of about 10 to 1, dry weights and inthe absence of any dispersing agent, (c) drying the blend and heating toabout 800 C., and (d) cooling and pulverizi-ng the mass.

This invention relates to an improved alumino-silicate pigment forpaints, and a method of making same. I mportant properties of pigmentsfor white paint, especially for paints with a latex base, are whiteness,brightness and hiding-power, the last named property being of particularimportance as a measure of the ability of a paint film to cover andconceal a dark surface.

The present discovery is a new development in a process which began withthe discovery of a novel and effective way to delaminate large kaolinparticles into platelets, such process being described in US. Patent No.2,904,267 granted to Lyons Sept. 15, 1959. The next step was theproduction of a kaolin product of superior whiteness by fractionatingwashed clay, delaminating the coarse fraction having not over aboutfines, and fractionating the treated clay to recover the delaminatedfraction, these particles being characterized by having freshly exposedwhite surfaces which give the product a high degree of whiteness.

It is well known that the brightness of kaolins can in general beimproved by calcining, usually at a temperature of about 1000 C., asthis temperature has been considered necessary to develop adequatebrightness. This fact has been recognized, for example, in the recentpatent to Gunn and Morris, No. 3,171,718, column 21, where it is statedthat in general, the clay fraction is calcined by heating to atemperature range of about 980 C. to 1038" C. This treatment, however,produces an abrasive quality in the clay which makes it unsuitable formany uses such for example as a filler or coating material for paper.

In experimenting with delaminated kaolin particles having freshlyexposed white surfaces, we have found that adequate brightness can behad by heating the material to a temperature of only about 800 C. ormuch less than the customary 1000 C. The new pigment thus prepared hasbeen found to yield paints with excellent qualities of brightness,whiteness and hiding-power when compared with the best alumino-silicatepigments heretofore known. When this effect was first noted, it wasassumed that it was due to the superior whiteness of the delaminatedkaolin, but when such kaolin was used as a pigment without first beingheated to about 800 C. it was found to F 5 3,403,041 Ice Patented Sept.24, 1968 have even less hiding power than natural kaolin ofcorresponding size distribution.

One practical drawback to this pigment (i.e., freshly delaminatedparticles heated to about 800 C.) was the tendency on the part of manyof the particles to agglomerate in the calcining process and not toseparate when the calcined clay was quenched in water in order toprepare it for reclassification. To inhibit this tendency toagglomerate, a small percentage (7% to 10%) of fine crystallizedaluminum trihydrate was added to the clay before heating it. To beeffective, this material must be added to the clay in a water slurrywithout the customary addition of a dispersing agent. When this mixturewas filtered, dried, pulverized, heated to about 800 C. and quenched,the resulting product was found to have superior qualities as a pigmentfor paint in hiding power, whiteness, brightness and flatting elfect.

On the drawings:

FIGURE l'is a flow diagram showing the process steps for obtaining ourimproved product; and

FIGURE 2 is set of graphs made by a recording spectrophotometer to showthe reflectance properties of four kinds of kaolin when individuallymixed with tricresyl phosphate (TCP), a colorless liquid having arefractive index 1.56 which is almost identical with that of kaolin.

The process indicated in FIGURE 1 starts with kaolin which has beendelaminated by the extrusion process described in US. Letters Patent No.2,904,267. The clay is slurried in water without the addition of anydispersing agent and to it is added an aqueous slurry of fine aluminumtrihydrate, i.e., Al(OH) the additive material being about 10% of theclay, by dry weight. The mixed slurry is dewatered by filtration,sedimentation or otherwise, then is dried and preferably pulverized. Thedry powder is heated to a temperature of from 750 C. to 900 C., carebeing taken to see that the temperature does not get anywhere near thecustomary 1000 C. The powder when heated to about 800 C. is then cooledeither by quenching in water or by air-cooling. If quenched in water,the resultant slurry is classified for the elimination of oversizeparticles, e.g., particles larger than 10 microns, BSD, and the residueis dried and pulverized by a hammer mill or other suitable apparatus.

While there is not yet complete understanding, nor full agreement, as tothe exact nature of all the various changes which occur when a kaolinitecrystal is heated progressively from room temperature to 1000 C., it isknown that one of the results of this treatment is the completedestruction of the parent kaolinite molecular configuration, withresultant formation of :large numbers of hard needle-shaped crystals ofmullite (having a square cross-section). If delaminated kaolin slurry ismixed with about 10% (dry weight) of aluminum trihydrate slurry andheated to about 800 C. as hereinbefore described, partialrecrystallization of the kaolin takes place with a minor proportion ofinterspersed aluminum oxide platelets and lange numbers of incipientmullite needles intercrystallized between the confronting faces of thealuminum oxide particles and the recrystallized kaolin particles, theneedles being embedded in a matrix of crystoballite which is a form ofsilica.

These incipient mullite needles, as formed at lower temperatures, e.g.,800 C., are very much smaller than are the parent kaolinite crystalsfrom which they derive. Electron micrographs of fully formed mulliteneedles v fr f 1 have been obtained which show them to be often arrangedin beautiful configurations reminiscent of a basket-weaving, with 120angles between them. One such micrograph was published, facing Page 16in Eighth NatiOnal Conference on Clays and Clay Minerals, 1960, NewYork, Pergamon Press, and this remarkable orientation of the needlessuggests that they may have grown from rows of aluminum atoms inhexagonal atomic arrangements in the original kaolinite unit cells.

We have found that mullite needles are of extremely smallcross-sectional dimensions when thekaolin is fired to low temperatures,e.g., 800 C., but that they become larger and fewer as the firingtemperature is increased and protrude from the surfaces of the parentparticles. It is believed that this accounts for the well-known abrasivequality of ordinary calcined kaolin.

It seems probable that the superior hiding power of our novel pigmentmay derive, at least in part, from the formation of very fine acicularcrystals within the parent larger kaolinite crystal, such as theseduring the recommended firing range.

FIGURE 2 shows the color anlysis curves developed on a recordingspectrophotometer for four clay samples each of which had been made intoa paste with tricresyl phosphate (TCP). The four clay samples areindicated as follows:

Vnatural washed kaolin,

V-800natural washed kaolin which has been heated to about 800 C.,

Sartificially delaminated kaolin,

S-800artificially delaminated kaolin which has been heated to about 800C.

These curves indicate a number of interesting properties of the sampleswhen compounded in a liquid of high refractive index, e.g., tricresylphosphate. For example, the sample Swhile it is the whitestis,nevertheless, the least opaque and, in a sense, is the most translucent,whereas the sample V is the more opaque of the unheated samples, thoughits whiteness is not good.

However, the 800 C. calcining produces remarkable and divergent changesin these optical properties.

Samples 8-800 becomes more opaque and reflective at 457 Tl'l/L, and itswhiteness sufiers only moderately, whereas V-800 becomes much morereddish and its reflectance at 457 m is materially decreased. Thewhiteness in each case is indicated inversely on the chart by thevertical distance between the curve point at 700 millimicrons and thepoint at 400 millimicrons, a shorter such distance indicating a whitersubstance. Perfect whiteness would be represented by a horizontal line.The values of brightness and whiteness of the samples indicated on thechart are:

Sample Brightness at 457 m whiteness index, 700 Il1;1400 111 The graphs,however, do not indicate a very important property of the substances,namely, hiding-power when incorporated in a paint, especially a painthaving a latex base. To test for this property, a pigment is blendedwith TiO casein and latex, some other minor ingredients usually beingincluded. A generally used formula for a paint by which pigments aretested is as follows:

Grams Pigment to be tested 100 TiO (R-901) Casein, 15% solution 75Tetrasodium pyrophosphate (dispersant) 0.7 Phenylmercuric acetate(preservative) 1.0 Lecithin (soya derivative) 1.5 Defoamer 581B(optional proprietary additive) 1.5 Latex (butadiene-styrene) Genflo 67137.0 H O... 137.0

These components are thoroughly blended by sufiicient stirring and'thenare screened through a fine sieve.

The paint is applied by an applicator toform a film on a Morest chartwhich is-made of strongcardboard faced with a coated surface half ofwhich is very black, the other half being very white. When the paintfilm, uniformly applie dto both halves, has dried, a direct comparisonof the halves indicates visually or instrumentally the hiding power ofthe paint.

Aluminum trihydrate in pigment form is very white, very bright and has ahigh gloss, but its hiding power is poor. Blendings of variouspercentages up to 20% of'this trihydrate with delaminated clay, followedby heating to 800 C. indicated that maximum hiding power was had whenabout 10% of the trihydrate was mixed with the clay. As the sheen wasnearly constant for blends from 3% to 20%, the maximum hiding powercould thus be had without excessive sheen, this combination ofproperties being desirable for most house painting.

While the new pigment consisting of a blend of delaminated kaolin withabout 10% aluminum trihydrate, mixed in slurry form with no dispersingagent, dried and heated to about 800 C. is not quite as bright as pureTiO yet when incorporated as a pigment in paint, its brightness,whiteness and hiding power were found to be superior to a paint in whichpure TiO was the pigment, and it gave a much better flatting effect,that is, producing a low degree of sheen or gloss, the resultant matsurface being often a desired finish.

We claim:

1. The method of preparing an alumino-silicate pigment of superiorwhiteness, brightness and hiding-power, which comprises delaminatingwashed kaolin, recovering the fines fraction up to 10 microns (E.S.D.),making an aqueous slurry of the delaminated kaolin, making an aqueousslurry of aluminum hydrate, mixing together the two slurries in theproportion of one part by dry weight of aluminum hydrate to aboutl0-parts by dry weight of kaolin, dewatering and drying the blendedslurry, heating the mixture to a temperature of about 800 C.-and coolingand pulverizing the calcined mass.

2. The method of preparing an alumino-silicate pigment as described inclaim '1, the calcined mass being cooled by quenching in water,fractionating the mass to eliminate oversize particles, and drying andpulverizing the residue.

References Cited UNITED STATES PATENTS 2,904,267 9/1959 Lyons 106723,021,195 2/1962 Podschus et al 23-1102 3,151,993 10/1964 Bundy 1063083,223,546 12/1965 Hemstock 106288 JAMES E. POER, Primary Examiner.

